Like other integrated circuit structures, micromechanical systems (MEMS) are also commonly formed on a silicon substrate, however, in the case of MEMS the entire thickness of the wafer is used for forming the micromechanical structure. The MEMS are formed by repeated deposition of layers or films, followed by patterning and etching to define the desired circuits. The result is that the layers or films formed in MEMS technology are typically thicker than in non-MEMS integrated circuits, with the effect that the backend processing associated with MEMS results in bowing of the wafer, which in turn is associated with stress produced in the films. For instance Damascene processing and ferromagnetic laminations formed in MEMS technology can produce significant bowing of the wafer.
It is therefore important to be able to measure the stress in these films. One approach that has been used is to measure the amount of bow of the wafer by making use of a laser scanner. A laser beam is typically scanned across the wafer, incident at some angle, and the deflection of the laser is measured. A bowed wafer will deflect the laser beam differently to that of a flat wafer. Insofar as there are no features on the wafer that will cause the laser light to scatter, this approach works pretty well. However, when wafers are patterned to define certain features on the wafer, this approach becomes problematic. In particular, this prior art technique to measuring film stress is limited to measurements where the stressed film maintains a planar configuration in the sense that it does not involve deposition within trenches or on sidewalls of features.
The present invention seeks to address some of these issues.